The invention relates to methods of, and apparatus for, regenerating ion exchange materials.
Modern high pressure boilers require a very high degree of purity in their feed water, particularly boilers of the once-through type. It is essential to ensure that corrosion products do not enter the boiler system and also to guard against ingress of soluble compounds due to condenser leaks and other faults.
Very high purity water is often required in other industries also, for example as wash water in the electronics industry for washing electronic components which have to be absolutely free from impurities during manufacture.
One of the most important water treatment processes for achieving such high purity water is the mixed bed deionisation process. The use of a mixed bed of ion exchange material means that, in effect, the water is passed through a very large number of cation and anion layers.
The regeneration of such mixed beds requires that the ion exchange materials are separated into layers. This is achieved by backwashing the mixed materials to classify them and to cause the anion material, which has the lower density, to rise to form an upper layer resting on top of the cation material.
After separation of the materials into their respective layers, the anion and cation materials can be regenerated using sodium hydroxide and sulphuric or hydrochloric acid, respectively.
It is at this stage where imperfections in the process arise. For example, at the interface between the layers, it is impossible to achieve perfect separation of the materials and consequently each layer is contaminated to some degree by material of the other layer. For the maximum degree of purity of the treated water it is important that mixing of one type of ion exchange material with another should be eliminated as far as possible.
The reason for this is that any cation material mixed with anion material is contacted, on regeneration, by the sodium hydroxide regenerant which causes the cation to be converted to the sodium form. This sodium form of the cation can subsequently give rise to sodium leakage during service flow through the mixed bed.
In the case of anion material the position is more complex. It is recognised with the types of anion materials currently available that, during their life, degradation takes place and some of the strong base groups are degrated to weak base groups. Thus, if anion material is left in the cation material, the weak base groups are converted to the sulphate form which during the subsequent treatment cycle are hydrolysed to the free base with the release of sulphuric acid. In addition strong base groups are converted to the bisulphate form and it has been found that anion resins will also absorb sulphuric acid. The rate at which bisulphate form of anion resin hydrolyses to give the sulphate form and the release of sulphuric acid and/or the rate at which absorbed acid is released appears to deteriorate with age of resin. This results in anion material increasingly retaining the acid during the standard rinsing period thus leaving more to be leached out during service flow. The absorption of acid by anion resins also applies when hydrochloric acid is used to regenerate the cation resin, thus giving the release of hydrochloric acid into the water being treated when the anion resin has aged.
The separated layers of ion exchange material can be regenerated in the vessel in which they are separated, the respective regenerants being fed into or taken from the vessel at a distributor/collector means positioned at an intermediate position of the vessel. A typical regeneration method of this type is described in UK Patent Specification No. 1318102, dated Nov. 23, 1970.
In this type of method, it will be clear that even when the interface between the layers is coincident with the distributor/collector means, because of the limitations on the definition of the interfacial region, some material from each layer will be contacted with the incorrect regenerant. In practice, it will be very difficult to ensure that the interfacial region is coincident with the distributor/collector means and consequently relatively large amounts of one or other of the layers may contact the incorrect regenerant.
The separated layers may be isolated from one another, for example by the anion layer being transferred to another vessel, prior to being regenerated. A typical regeneration method of this type is described in U.S. Pat. No. 3,414,508 to Applebaum et al, issued Dec. 3, 1968,
The success of that type of method, however, depends on the accuracy of the definition of the interface; on the closeness of said region to an outlet for the anion layer; and on the degree to which any turbulence of the transfer water has caused mixing of the layers in said region during the transfer step. As cation contamination of the anion layer has been regarded as the more serious of the two situations, it has been the practice to position the outlet so that only anion material has been transferred, even at the expense of leaving anion material in the cation layer.
A further proposal for achieving separation of the layers is described in U.S. Pat. No. 4,120,786 to Peterson et al, issued Oct. 17, 1978, which describes classifying the anion and cation materials into two distinct layers. Once classification has been achieved, the upward flow of classifying water is maintained while cation material is drained from the base of the vessel in which classification has occurred. The continued upward flow of water together with the shape of the conical fluid distributor is alleged to result in perfect separation of the materials. Once the cation material has left the vessel, a valve is closed to retain the anion material in the vessel and it is alleged that no cross-contamination of materials occurs.
In the applicant's experience, however, even with very careful hydraulic classification of the materials continued for the maximum practicable time cross-contamination of the materials is unavoidable. In the method disclosed in U.S. Pat. No. 4,120,786 some water must flow out of the vessel with the cation material and it is believed that the interface between the two layers must be disturbed by such flow and that cross-contamination of the materials is thereby increased.
That method necessarily imposes very critical limits on the design and positioning of the valve and the pH monitor, which is used to detect the interface.
A further known practice has been to raise the pH of boiler feed water to e.g. 9.4-9.6 using ammonia, which reduces corrosion in the boiler. Ammonia is preferred because it passes through the vapour cycle and re-dissolves in the condensate. To stop the cation material stripping ammonia from the boiler water, the cation material is ammoniated after being regenerated. This ammoniation step can also be applied to the regenerated anion layer to convert the sodium form of the contaminant cation material to the required ammoniated form as described in U.S. Pat. No. 3,385,887, issued May 28, 1968.
This practice, however, only provides a solution to the problem and does not prevent it; nor is it a solution when ammoniation of the treated water is not required and may even be undesirable.
Alternatively, as that process uses a considerable quantity of ammonia it is usual to operate initially with the hydrogen form of the cation material thus allowing the cation material to strip ammonia from the condensate. The process requires ammonia to be re-introduced downstream of the ion-exchange units to maintain the required pH level.
However, when all the hydrogen sites on the cation material are exhausted by ammonia, the ammonia then displaces any sodium thereon from the cation material and leads to sodium leakage into the boiler water.
Clearly, the amount of leakage is dependent on the amount of sodium remaining on the cation material after regeneration which, in turn, depends on the separation achieved during classification and transfer or regeneration and the efficiency of regeneration.
In a similar manner, chloride leakage may occur due to the displacement of chloride ions from the anion material by hydroxide ions. This again depends on the separation achieved during classification and transfer or regeneration and the efficiency of the regenerant.